The stratigraphy, structure and regional significance of the Moine Rocks of Mull, Argyllshire, W. Scotland

Abstract

AbstractTwo right‐way‐up Moine lithostratigraphic units—the Shiaba (older) and Assapol (younger) Groups—are distinguished in the Ross of Mull. These were intruded in the west by the post‐tectonic, Ross‐of‐Mull granite complex at 414 ± 3 Ma. Apparently undisturbed inclusions of Moine metasediments within the granite permit the boundary between the lithostratigraphic units to be followed westward almost to the supposed trace of the Moine thrust in the Sound of Iona. The Shiaba and Assapol Groups which have a transitional, albeit attenuated, stratigraphic contact are correlated with the Morar and Glenfinnan Divisions of Inverness‐shire, the Sgurr Beag ductile thrust (slide) normally found at the Morar/Glenfinnan Division boundary being absent. This implies that the stratigraphic relationship between these two divisions, which is elsewhere obscured by the thrust, is uniquely preserved on Mull.Within a local, D1‐D4 sequence, D2 and D3 structures are dominant. Originally subhorizontal D2 folds are intensely curvilinear on all scales about an originally flat‐lying, NNW‐SSE trending stretching lineation (L2). In sections parallel to L2, D2 minor folds display a constant sense of vergence throughout the Ross‐of‐Mull Moine Rocks, overturning generally NW‐NNW. The present day structure is dominated by the almost upright, SSW‐plunging D3 Assapol synform which overprints all earlier structures. Tentative correlation of the deformation sequence with that of Inverness‐shire, suggests that the D1‐D2 structures of Mull, with accompanying moderate‐to‐high‐grade metamorphism, may be Precambrian, while the D3 Assapol synform may be Caledonian.The presence of migmatites of kyanite grade means that metamorphic grade, established during MS1‐MP2, is anomalously high for Moine rocks lying close to the supposed Caldonian front. This suggests that they may lie within one of the higher Caledonian thrust nappes of the North Highland Moine—possibly the Knoydart nappe, where metamorphic grade is comparable. The greenschist facies metamorphism and single phase of deformation affecting the ‘Torridonian’ rocks of Iona presents a significant contrast to the Moine rocks of the Ross of Mull. A major fault in the Sound of Iona is implied, but the Moine thrust itself probably does not outcrop.